As years went by, not only did the waterways become congested, but the roadway above began to be encroached on by houses and other buildings, for which a bridge was most unfitted. Two edifices, however, from the first formed necessary, or at least customary, adjuncts to such a building--the bridge gate and the bridge chapel. It was a Roman custom to erect gates at one end, or in the centre of their bridges--not triumphal arches, but twin gates, such as they built to their walled towns, and such as stood at the end of the bridge at Saintes, when it was altered by Isembert. Such gates as survived in mediæval times were generally fortified, and formed the model for imitation by mediæval builders; and such a gate was erected at the Southwark end of London, which, under its name of Bridge Gate, became one of the principal gates of the city. It was erected directly on one of the main piers, and was therefore of a substantial character, but suffered much in the various attacks made upon London from the Kentish side. In 1436 it collapsed, together with the Southwark end of the bridge, but was rebuilt at the cost of the citizens, chief among whom was Sir John Crosby, the builder of Crosby House; and although the gate was again in great part destroyed by the attack on London made in 1471 by Fauconbridge, one of the towers of Crosby's gate survived until the eighteenth century. In 1577 the tower which stood at the north end of the bridge, and on which were usually displayed the heads of traitors, became so dilapidated that it was taken down, and the heads then on it were transferred to the Bridge Gate, henceforward known as "Traitors' Gate." It was upon the earlier gate that the head of Sir Thomas More was affixed, when heads were so common that even his, as we know from its adventures until buried in the Roper vault at Canterbury, was thrown into the river to make room for a crowd of successors. [Illustration: FIG. 2--THE SURREY END OF LONDON BRIDGE.]